3Com VCX V7122 SIP VoIP Gateway User Manual 145The ISDN Non-Facility Associated Signaling (NFAS) feature enables use of a single D-channel to control multiple PRI interfaces.With NFAS it is possible to define a group of T1 trunks, called an NFAS group, in which asingle D-channel carries ISDN signaling messages for the entire group. The NFAS group’sB-channels are used to carry traffic, such as voice or data. The NFAS mechanism alsoenables definition of a backup D-channel on a different T1 trunk, to be used if the primary D-channel fails.The NFAS group comprises several T1 trunks. Each T1 trunk is called an ‘NFAS member’.The T1 trunk whose D-channel is used for signaling is called the ‘Primary NFAS Trunk’. TheT1 trunk whose D-channel is used for backup signaling is called the ‘Backup NFAS Trunk’.The primary and backup trunks each carry 23 B-channels while all other NFAS trunks eachcarry 24 B-channels.The VCX V7122 supports multiple NFAS groups. Each group should contain different T1trunks.The NFAS group is identified by an NFAS GroupID number (possible values are 1, 2, 3 and4). To assign a number of T1 trunks to the same NFAS group, use the parameter‘NFASGroupNumber_x = groupID’. ‘x’ stands for the physical trunkID (0–7).The parameter ‘DchConfig_x = Trunk_type’ is used to define the type of NFAS trunk.Trunk_type is set to 0 for the primary trunk, to 1 for the backup trunk and to 2 for an ordinaryNFAS trunk. ‘x’ stands for the physical trunkID (0–7).For example, to assign the first four VCX V7122 T1 trunks to NFAS group #1, in which trunk#0 is the primary trunk and trunk #1 is the backup trunk, use the following configuration:NFASGroupNumber_0 = 1NFASGroupNumber_1 = 1NFASGroupNumber_2 = 1NFASGroupNumber_3 = 1DchConfig_0 = 0 ;Primary T1 trunkDchConfig_1 = 1 ;Backup T1 trunkDchConfig_2 = 2 ;24 B-channel NFAS trunkDchConfig_3 = 2 ;24 B-channel NFAS trunkThe NFAS parameters are described in Table 29 on page 127.NFAS Interface IDSeveral ISDN switches require an additional configuration parameter per T1 trunk, that iscalled ‘Interface Identifier’. In NFAS T1 trunks the Interface Identifier is sent explicitly inQ.931 Setup / Channel Identification IE for all NFAS trunks, except for the B-channels of thePrimary trunk (see note 1 below).The Interface ID can be defined per each member (T1 trunk) of the NFAS group, and mustbe coordinated with the configuration of the Switch.The default value of the Interface ID is identical to the number of the physical T1 trunk (0 forthe first VCX V7122 trunk, 1 for the second VCX V7122 T1 trunk etc. up to 7).To define an explicit Interface ID for a T1 trunk (that is different from the default), use thefollowing parameters:
